Professor Ding's primary research interests include design and implemenetation of optoelectronic and nonlinear optical devices for generation, amplification, and modulation of tunable and coherent waves in the domains of UV, blue, green, mid-infrared, THz, and submillimeter.

Our eventual goal is to efficiently generate and amplify terrahertz (THz) waves, to implement transversely-pumped counter-propagating optical parametric oscillators and amplifiers, and intersubband lasers based on a variety of novel structures and configurations.

On the road to our objectives, we have considered forward and backward optical parametric oscillation and amplification, and difference-frequency generation for efficiently generating and amplifying terahertz waves in CdSe, GaSe, periodically-poled LiNbO₃ and LiTaO₃, and diffusion-bonded-stacked GaAs and GaP plates. The advantage of using birefringence in CdSe and GaSe is tunability of the output terahertz frequency. Furthermore, both CdSe and GaSe can be used to achieve the backward parametric oscillation without any cavity. On the other hand, in periodically-poled LiNbO₃ and LiTaO₃, one can take advantage of large diagonal elements of second-order nonlinear susceptibility tensor. In the diffusion-bonded-stacked GaAs and GaP plates, quasi-phase matching can be achieved by alternatively rotating the plates. The advantage of using coherent parametric processes is possibility of efficiently generating and amplifying temporal-coherent and narrow-linewidth terahertz waves. Compared with a noncollinear configuration, by using the parallel wave propagation configurations, the conversion efficiency can be higher because of longer effective interaction length among all the waves.

Our motivation is to use KTP crystals for intracavity frequency-doubling 1.064-mm beam. We have developed a simple method for measuring damage threshold of KTP crystal for CW irradiation using an Argon laser. The experimental results show that there are two types of optical damage in KTP crystal depending on the polarization of the incident laser beam. One type of optical damage corresponds to gray tracks that are formed when the polarization is perpendicular to the z-axis. Another one is invisible damage that occurs when the polarization is parallel to the z-axis. In addition, we also observed photorefractive two-wave mixing in KTP crystal under each of the above two polarization states. Our experimental results imply that there exists charge drift during the process of optical damage at both of these polarization states, but the mechanisms are different at these two orthogonal polarization states. After analyses, we conclude that the first type of the damage is due to the formation of Ti3+ centers and the second one is due to the drift of K+ ions. In these KTP crystals, we have generated blue light by second-harmonic generation in one KTP crystal and subsequent sum-frequency generation in second Ce-doped KTP. The latter crystal was used to reduce the absorption in the blue region as a result of the doping. The overall efficiency achieved by us is around 2%. We have characterized ion-exchanged KTP waveguides using backward second-harmonic generation. Because of extremely short periods of the index gratings in each waveguide, we observed backward second-harmonic generation at the 6th and 7th orders. These phase-matching peaks are accompanied by the presence of other sharp peaks due to distributed Bragg reflection. Although the conversion efficiency is low, we should be able to improve it to a level for many practical applications by optimizing the designs including achieving domain inversion. We have determined the mean period fluctuation from our measurements.

We have observed high-order phase-matching second-harmonic generation peaks in the reflection geometry in GaAs/AlGaAs mutlilayers. We have designed an optimized structure that can be used to generate coherent beam at 3.2 micrometers by mixing 1.06 and 1.6 micrometers in a GaAs-based mutilayer structure. We have also designed and grown another optimized structure for generating 2.7 micrometers by mixing 980 nm from a laser diode and 1.55 micrometers from an erbium-doped fiber laser. We have shown that these structures can be used as efficient optical amplifiers and frequency shifters.